When Dr Jacques Bérès crossed into Syria by truck last week, his hulking suitcase full of surgical kit was perched against an awkward cargo – two dozen rocket launchers.

The retired French surgeon – who has volunteered his services in nearly every major global conflict since Vietnam in 1968 – said he rarely had to share transport with gunrunners on his mercy missions. But nothing about this war in Syria seems to be going to script.

"It's not good," Bérès said of his arrival. "In principle, it is forbidden for humanitarian people to travel with weapons. But it is their country and their war. We are the observers. We are just here to help in some way."

But on Saturday, even as diplomats sought UN backing for an Arab plan to end the bloodshed, reports came from the state-run news agency that a senior army general had been assassinated in Damascus, the first killing of a military figure in the Syrian capital since the uprising began in March last year.

As Syrian forces continued the week-long siege of Homs with a rocket bombardment of its opposition neighbourhoods, violence reaching the capital was a new development.

The UN estimates that 5,400 people have been killed in Syria since the uprising began, but it stopped counting in January and hundreds are reported to have died since.

In the three days that the 71-year-old orthopaedic surgeon Bérès has been near Homs, he has been at the centre of an escalating uprising. Hours after arriving on Thursday he helped to save the life of a gunshot victim and gave first aid to five seriously wounded opposition fighters.

On Saturday he operated on a civilian shot in the leg, as the victim's family and Free Syrian Army soldiers waited anxiously outside. The fighting has seen opposition fighters launching attacks last week against key government posts.

But the destruction during the past week of the two most prominent resistance hubs in Homs, Baba Amr and al-Khalidiyah, had its effect in the city's outskirts, where residents are waiting for an invasion.

"It's 100% certain that they will do the same here that they have done in Homs," said Abu Mahmoud, a Free Syrian Army captain, as he arrived at a clinic. "We know they are coming and we are preparing for them. We only have light weapons," he said, pointing at the webbing around his waist that carried five ammunition clips for a Kalashnikov and a hand grenade.

Only six weeks ago, this hard-bitten rebel was a career officer in the Syrian army. "But they wanted us to kill our own people, our own families," he said, standing in the muddy courtyard of the improvised clinic. "I waited for the chance to run. There were a group of officers who they thought were going to escape and a military firing squad killed 17 of us. I got away."

Abu Mahmoud is a recent defector; he waited for almost 10 months before fleeing and was party to some of the most prominent operations of the regime crackdown, in Idlib, Deraa and Homs. But the time it took him to defect is not being held against him in his home town, where he is now one of the Free Syrian Army's local leaders.

"Every officer like him had three people from Assad's army watching him," said the lead physician at the clinic, Dr Qassem. "He couldn't run. If he did, he would have been killed."

Captain Mahmoud offered a warning: "In Homs they are firing from the hospital and other high ground. Here, they are only five or six kilometres away, in the military firing range. They have positions on every exit from town and some units are less than one kilometre away."

He picked up a box of medicines, turned for the gate and left.

Minutes later the wounded civilian arrived, blood pouring from a bullet wound above his right knee. Dr Bérès has personal experience in treating such injuries; he has been shot three times himself. One bullet in Monrovia claimed a finger, another in Chechnya caused a deep wound to his side, and a third in Sudan left his right arm scarred. "It is normal to treat such things," he said. "Very normal. I have been doing it all my life."

His war wounds have won him kudos among the band of medics at the clinic. All of them fled the nearby state-run hospital, which is now being used as a firing position by the Syrian army. "They know where we are and we are all wanted," said Dr Qassem. "I don't know why they haven't bombed us yet. We saw what happened to the clinics in Homs."

Frontline medics have been killed and wounded in Homs and their facilities and dispensaries have been hit by mortars and rockets. So, too, has a hospital. The opposition-held sectors of the city have been battered into submission by the eight-day barrage.

Few people are making it out of the satellite towns and villages that spill north to Hama, or south towards Lebanon. "We still can't get there," said one medical worker. We want to go very much, but the roads are not safe."

So, too, does Bérès. "The symbolism is very strong," he said of the presence of foreign doctors in a war zone. "People here are happy to meet a surgeon from a well-developed country who just wants to be with them. "It seems to be a war here, yes. But I don't know if it's a continuation of the (Arab) spring, or a religious war between the Alawite and the Sunni people."

For Bérès, some 44 years in the field, including 10 trips to the war zones of Lebanon, Gaza, the Balkans, Ivory Coast, Afghanistan and Iraq, interspersed with time at Paris hospitals, have chipped away at the ideals he first brought to the profession. "Humanity is not drifting away, but it's not improving," he said.

As he prepared to move on into Homs itself, which could for him be perhaps his last stop on a long road of humanitarian help, he delivered a prediction that seemed to echo around this place, in the hinterland of civil war. "I am not optimistic about Syria," he said gravely. "It is a very difficult situation."

• This article was amended on 16 February 2012 to remove the name of a town.